Vehicle detectors are key components in all street and freeway traffic control and surveillance systems. An ideal detector for these applications should be low in cost, provide accurate detection, require minimum installation time and cost, be reliable under all environmental conditions, have low maintenance and calibration requirements, and be able to detect all vehicles on any standard roadway surface.
The United States Navy has developed and patented (U.S. Pat. No. 4,302,746) a self-powered vehicle detection (SPVD) system for the Federal Highway Administration. The SPVD system detector includes a two-axis magnetometer that measures a motor vehicle's magnetic signature. The signature is processed to determine vehicle presence and is then transmitted to a road-side receiver system. The operating principle of the SPVD is to sense the magnetic field of the vehicle and transmit a leading and trailing edge signals corresponding to magnetic signature threshold levels. Since the magnetic field signature amplitudes vary with respect to the size and shape of motor vehicles, the speed of a motor vehicle must be determined using two precisely spaced SPVD detectors or other current state of the art speed sensors (eg., loop detectors). Unfortunately, the process of burying a plurality of SPVD detectors and/or loop detectors in a roadway is time consuming and costly.
In addition, the amount of magnetic material used in motor vehicles has decreased over the last ten years. A recently built motor vehicle's magnetic field signature amplitude is less than that of a comparably sized motor vehicle built a decade ago. Therefore, today's highway vehicle sensing system based on magnetic field signatures requires a greater sensitivity to detect smaller amplitude magnetic signatures.